1. Field of the Invention
The present invention relates to an electronic delay circuit for firing an ignition element, which stores electric energy supplied from an electric blasting machine, and fires a detonator at high accuracy following a predetermined delay.
2. Related Art
A method for controlling ground vibrations to minimize the effect of blasting on nearby structures in a multi-step blasting system is proposed by Japanese Patent Application Laid-open No. 285800/1989. This application teaches the accuracy of a delay time interval for sequentially firing detonators in order to reduce the ground vibrations. According to the application, it is preferable that the delay time interval t and the standard deviation .sigma. of the delay time interval t satisfy the following relationship: EQU t/.sigma..gtoreq.10 (1)
The application states that unless the condition is satisfied, the ground vibrations may not be reduced.
Consequently, in order to set the delay time interval for firing detonators at 10 ms, the standard deviation of the delay time must be equal or less than 1 ms. Likewise, the delay time interval of 5 ms requires the standard deviation of 0.5 ms or less.
Japanese Patent Application Publication No. 53479/1988 discloses an electric detonator fired by an electronic delay circuit. This circuit receives only electric energy via leading wires, activates a digital timer comprising a quartz or ceramic oscillator, and fires an electric detonator after a predetermined delay time. The application, however, does not teaches a technique to satisfy the condition of equation (1).
U.S. Pat. No. 4,445,435 teaches an electronic , delay blasting circuit which comprises a means for storing electric energy, an oscillating circuit using a quartz or ceramic oscillator, a digital timer including a counter and a counter reset circuit for resetting the counter, and a means for firing an electric detonator after a predetermined delay time.
Likewise, European Patent Application Publication No. 261,886 discloses a delay circuit for electrically firing a detonating primer (detonator).
FIGS. 1 and 2 illustrate the principle of a delay circuit disclosed by the European patent application. In this figure, an electric blaster (electric blasting machine) 1 supplies a voltage (electric energy) as shown at (a) of FIG. 2. The electric energy is supplied to an actuation circuit 2, a capacitor 3, a clock pulse generator 4 and a counting circuit 5 via leading wires 6, and is stored into the capacitor 3 as shown at (b) of FIG. 2. The actuation circuit 2 maintains the reset state of the counting circuit 5 during counter reset time T (=200-300 ms), and actuates the counting circuit 5 to start counting when the counter reset time T has elapsed after the application of the input voltage is initiated. The time T is defined by the falling edge of the input voltage as shown at (a) and (c) of FIG. 2. This is because the output frequency of a quartz or ceramic oscillator included in the clock pulse generator 4 becomes stable after the counter reset time T as shown at (c) of FIG. 2. The counting circuit 5 counts the pulses of the pulse train produced by the clock pulse generator 4, and trigger a switching circuit 7 as shown at (d) of FIG. 2, so that a current is supplied from the capacitor 3 to an ignition circuit 8. Thus, an electric detonating primer (detonator) is fired after a predetermined delay time as shown at (e) of FIG. 2.
The quartz or ceramic oscillator employed by the clock pulse generator 4 has a problem in that the oscillator takes approximately 200-300 ms before it enters into a steady state oscillation. In other words, the output frequency thereof is unstable during the counter reset time T. Accordingly, the prior art cannot start counting pulses of the pulse train outputted from the oscillator until the counter reset time T has elapsed.
The long counter reset time T presents a problem in that the delay time becomes unstable. There are two major reasons for this.
First, as the counter reset time increases, the probability that the input voltage as shown at (a) of FIG. 2 may be influenced by external noise during the counter reset time T increases. The external noise may change the falling edge of the input voltage, and hence will change the starting time of the counting circuit 5. This is a great problem because there is much noise caused by incomplete contact of the lines (leading wires) or by switching the electric blasting machine or the like in a blasting site. To improve such a problem, the actuation circuit 2 becomes complicated, and hence increase in size and cost thereof will be unavoidable.
Second, as the counter reset time increases, the error of the counter reset time will increase since the counter reset time is specified by the analog voltage. In addition, the increasing electric power will be consumed as the counter reset time becomes longer.
In addition, it is difficult to apply the prior art to a serially connected blasting system.